Mossberg sells a line throwing adapter for their 500/590 series shotguns. Remington doesn't list any on the website but they do sell shotties specifically for marine environments so a call to the service dept may yeild results. I know you were wanting to improvise but I'm leery of sticking anything down the bore of a gun that I'm about to shoot (a ball down a muzzle loader being an obvious exception). Any online instructions or Paladin Press type books should be taken with a liberal dose of salt.

A cheaper and more compact alternative that could be easily duplicated at home can be found here www.ezhang.com

Ed